Contact potential difference ionization detector

ABSTRACT

A nondestructive testing method of condensed matter surfaces, and a sensing device for the measurement of the work function of the surface of a conducting or semiconducting sample. The sensing device includes an ionization chamber, a probe having a first surface, and a potential difference measurement circuit that is capable of measuring a difference in potential between the first surface of the probe and a surface made of another material to be tested. The ionization chamber produces ionized particles that travel out of an output of the ionization chamber and toward the probe. The probe is a non-vibrating probe having a first surface that is either a positively or negatively charged electrode. The measurement circuit of the present invention is capable of sensing the small amount of electrical current that the electrons and ions moving toward the first surface and the testing surface represent.

This invention was made with Government support under Grant NumberN00014-95-1-0903 awarded by the Department of the Navy. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a contact potential differenceionization detector and method of using such a detector to detect thework function of a surface, and more specifically to a probe fordetecting chemical changes on surfaces.

2. Description of Related Art

A Kelvin probe is a name given to a type of sensor that measures thedifference in work functions between the probe surface and a surface ofinterest (the testing surface). This measurement is made by vibratingthe probe and detecting an electrical signal related to the vibrationfrequency. The difference in work function between the Kelvin probesurface and the testing surface results in an electric field when thetwo surfaces are in electrical contact.

The work function of the surface of an electronic conductor is definedas the minimum amount of work required to move an electron from theinterior of the conductor to a point just outside the surface (beyondthe image charge region). If an electron is moved through the surfaceregion, its energy is influenced by the optical, electric and magneticcharacteristics of the region. Hence, the work function is an extremelysensitive indicator of surface conditions and is affected by absorbed orevaporated layers, surface reconstruction, surface charging, oxide layerimperfections, surface and bulk contamination, and other likeproperties. Thus, work function measurements are known and can beemployed for nondestructive evaluation of surfaces of various materials.See, for example U.S. Pat. No. 5,974,869 which is incorporated byreference herein in its entirety.

The traditional Kelvin probe incorporates a flat circular electrode(termed the reference electrode) suspended above and parallel to astationary electrode (the specimen), thus creating a capacitor. FIGS.1a-c illustrate various electron energy diagrams for two differentconducting materials. FIG. 1a shows the electron energy level diagramfor two conducting specimens that are electrically isolated from oneanother, where φ₁ and φ₂ are the work functions of the materials, and ε₁and ε₂ represent their Fermi levels. In this case, the Fermi energiesand work functions are referenced to a potential in the space betweenthe two materials.

In FIG. 1b it can be seen that if an electrical contact is made betweenthe two electrodes, their Fermi levels equalize by the flow of charge(in the direction indicated). The reference potential is no longer thespace between the materials, but is now referenced to the Fermi levels.Relative to the Fermi energy, there develops a potential gradient,termed the contact potential V_(c), between the electrodes. The flow ofelectrons to equilibrate the Fermi levels causes the two surfaces tobecome equally and oppositely charged.

Referring to FIG. 1c, the inclusion of a variable “backing potential”V_(b) in the external circuit permits biasing of one electrode withrespect to the other. At the unique point where the (average) electricfield between the plates vanishes, there is a null output signal. Thework function difference between two surfaces can be found by measuringthe flow of charge when the two conducting materials are connected (seeFIG. 1b). However, this produces a “once only” measurement as thesurfaces become charged, and the charge must dissipate before anothermeasurement can be made.

By vibrating one of the electrodes (the probe), as has been suggested byZisman and adapted by many researchers, a varying capacitance isproduced, and defined as:

C=Q/V=ε _(r)ε₀ A/d  (1)

Where

C is the capacitance;

Q is the Charge;

V is the Potential;

ε₀ is the permittivity of the dielectric (in an air probe the dielectricis air);

ε_(r) is the relative dielectric constant;

A is the surface area of the capacitor; and,

d is the separation between the plates.

If the vibration is periodic, then a periodic flow of electrons willresult to try to keep the Fermi levels of the two surfaces equal.Therefore, as the separation d increases periodically, the capacitance Cdecreases periodically.

As the probe oscillates relative to the testing surface, the voltagebetween the probe and the testing surface can be recorded. If thevibration is sinusoidal, then the peak-to-peak output voltage V_(p) isgiven by the equation:

V _(p)=(ΔV)RC ₀ωεsin(ωt+φ)  (2)

Where

ΔV represents the voltage between the Kevin probe and the sample;

R is the resistance of the measuring circuit;

C₀ is the Kelvin probe capacitance;

ω is the frequency of vibration;

φ is the phase angle; and,

ε is the modulation index (d₁/d₀) where d₀ is the average distancebetween the sample and the probe tip, and d₁ is the amplitude ofoscillation of the probe.

Yet, there are several disadvantages with capacitance probes like theKelvin probe as modified by Zisman. One problem with the modified Kelvinprobe is that the charge of the electrodes must be dissipated beforeanother measurement can be made, which limits the speed of operation ofthe probe. Another limitation of the modified or Kelvin type probearises when used in a gas environment. Measurement of the potentialdifference in a gas environment by capacitance probes presents problemsof reproducibility because adsorption of gas on a surface can causesignificant changes in the work function. Such adsorption affects notonly the samples being tested, but also the probe. A change in the workfunction of the probe is virtually indistinguishable from a change inthe work function of the sample. Other kinds of surface-gasinteractions, as well as changes in environmental conditions such asrelative humidity, also can strongly influence the measurements made byKelvin type probes and other capacitance probes.

Another serious limitation of Kelvin type probes is the need forvibration of the probe. The amplitude of vibration limits how close theprobe can be placed relative to the testing surface. The signal will berelated to this spacing; the closer the probe can be positioned, thegreater the signal and sensitively. Vibration of the probe also is asevere experimental constraints. It necessitates a power source andsystem to vibrate the probe, and the design of the geometry of thevibrating system. A sensing device that could overcome the manylimitations of conventional Kelvin type probes would be beneficial.

Thus, it can be seen that there is a need for the present invention, animprovement over the conventional capacitance probe, by providing acontact potential difference ionization detector that has no movingparts, yet is sensitive enough so as to be capable of sensing gascurrents due to the separation of ionized gases by the differences inchemical potential between two different metals. The present inventionis primarily directed to the provision of such a non-vibrating probe andits incorporation in a detector, where the signal is related to anionized gas in the gap of the probe surfaces.

SUMMARY OF THE INVENTION

Briefly described, in a preferred form, the present invention providesboth a nondestructive testing method of condensed matter surfaces, and asensing device for the measurement of the work function of the surfaceof a conducting or semiconducting sample. The present invention can beused to make extremely sensitive motion detectors and accelerometers.This methodology also can be applied to ascertain contact potential forselected non-metals to evaluate changes of chemical state at surfaces.

The present contact potential difference ionization detector comprisesan ionization chamber, a probe having a first surface, and a potentialdifference measurement circuit that is capable of measuring a differencein potential between the first surface of the probe and a testingsurface.

The ionization chamber produces ionized particles that travel out of anoutput of the ionization chamber and into the space between the probeand the testing surface. The probe is non-vibrating having a firstsurface that is either a positively or negatively charged electrode dueto the electrical connection between the first surface and testingsurface. The measurement circuit of the present invention is capable ofsensing the small amount of electrical current that the ions movingtoward the first surface and the testing surface represent.

Thus, an object of the invention is to provide an improved contactpotential difference ionization detector.

Another object of the present invention is to provide an improveddetector that can be used to make extremely sensitive motion detectorsand accelerometers.

Yet another object of the present invention is to provide an improvedextremely sensitive method of contact potential ionization detection.

These and other objects, features, and advantages of the presentinvention will be more apparent upon reading the following specificationin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates an electron energy level diagram for two conductingspecimens;

FIG. 1b illustrates the system of FIG. 1a with electrical contactbetween the two electrodes; and

FIG. 1c illustrates the system of FIG. 1b with an added backingpotential V_(b);

FIG. 2 is a diagram of a contact potential difference ionizationdetector according to a preferred embodiment of the invention;

FIG. 3 illustrates a preferred detection circuitry of the presentinvention;

FIG. 4 shows ion probe voltage values versus work function values for aseries of metals;

FIG. 5 shows a schematic of apparatus for measurement of ionizationprobe output arising from gas flow rate;

FIG. 6 shows a plot of measured ion probe voltage versus gas flow rate;

FIG. 7 shows a schematic diagram of the probe used to measure surfacevibration; and

FIG. 8 shows a plot of ionization probe output voltage versus spacingbetween the tip of the probe and a vibrating surface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now in detail to the figures, wherein like reference numeralsrepresent like parts throughout the several views, FIG. 2 shows acontact potential difference ionization detector 100 (hereinafter, the“detector 100”) comprising an ionization chamber 10, a probe 50 having afirst surface 60, and a potential difference measurement circuit 80(hereinafter, the “measurement circuit”) that is capable of measuring adifference in potential between the first surface 60 of the probe 50 anda testing surface 70. The testing surface 70 is only a part of the probe50 as it complements first surface 60 to form a two plate capacitor 75for the probe 50.

Gas molecules 35 that travel in the environment surrounding the detector100 enter into an input 20 to the ionization chamber 10 where theybecome ionized particles 40. The ionized particles 40 then travel out ofan output 30 of the ionization chamber 10 and toward the probe 50. Inone preferred embodiment of the ionization chamber 10, a source 25 ofionizing radiation for the ionization chamber 10 is an Americium-241source 87, and in other embodiments can be similar radioactive elements.Preferably, the amount of Americium-241 is approximately {fraction(1/5000)}th of a gram. The radioactive element Americium has a half-lifeof 432 years, and is a good source of alpha particles for the detector100.

The probe 50 of the detector 100 comprises a non-vibrating type of probewhich includes the first surface 60. The first surface 60 of the probe50 is part of a positively charged electrode 24, and a negativelycharged electrode 26 includes the second surface 70 which is thus anegatively charged surface. However, these charges can be reverseddepending on what materials are being used in the probe 50. The probe 50electrically connects the surfaces 60 and 70 so that the Fermi energiesare equal. The charges on the surfaces 60 and 70 occur when the surfaces60 and 70 are electrically connected and a difference in work functionsoccurs between them.

In one preferred embodiment, the electrode 24 (including the surface 60) is flat, as illustrated in FIG. 3, and the electrodes 24 and 26(including the respective surfaces 60 and 70) are, respectively, twodifferent conductive materials. Also shown are first diode 85 and seconddiode 86, along with voltage source 90, amplifier 88 and the 18 V powersource 92. The Americium source 87 ionizes the gas between the twosurfaces 60 and 70. When the metal electrodes 24 and 26 are connected,the contact potential produces an electrical field between the metalelectrode 24 and 26. The electrical field separates the ionized gas andproduces a voltage related to the contact potential difference, thenumber of ionized atoms and the gain of the amplifier 88. The voltagesource 90 also enables applying a bias potential to the first surface 60and the testing surface 70. The voltage output will also be related tothe number of gas molecules in the gap (between 60 and 70 in FIG. 3),the rate of ionization of these molecules and the rate at which themolecules strike the surfaces 60 and 70. As air or other gas moleculesmove into or out of the space between 60 and 70, the voltage will varyif all other parameters are kept constant. If motion occurs outside ofthe space between 60 and 70, this will cause gas molecules to flow intothe space. In this way, the non-vibrating ionization probe can serve asa motion detector. Such a motion detector would be very sensitive sincethe probe signals are very sensitive to the motion of gas molecules andthe number of gas molecules. This probe would be more sensitive thancurrently available motion detectors. Other applications of this motiondetector would involve the building of a housing to encapsulate theprobe so that only specific molecules can enter the region 60 and 70.Then the probe could discriminate between molecules. Alternatively, thepositively charged electrode 24 (including the surface 60) can varybetween numerous sizes and shapes that can be chosen by the user of thedetector 100 such that an electric field that can attract and repel theappropriately charged ionized particles can be generated by thepositively charged electrode 24 (including the surface 60) and thenegatively charged electrode 26 (including the surface 70).

The alpha particles generated by, for example, the Americium source 25in the ionization chamber 10 ionize oxygen and nitrogen atoms that areconstantly entering the input 20 of the ionization chamber 10. Suchionization provides a free electron (with a negative charge) and an atommissing one electron (with a positive charge), namely an ion. Thenegative electron is attracted to the positive electrode (including thesurface 60), while the positively charged atoms are attracted to thenegatively charged surface 70.

In a preferred embodiment of the circuit 80 shown in FIG. 3 themeasurement circuit 80 of the present detector 100 is capable of sensingthe small amount of electrical current embodied in the motion of theseelectrons and ions moving toward the surfaces 60 and 70. One importantfeature of the present detector 100 is the sensing of signals from thenon-vibrating probe 50, wherein the signal is related to an ionized gaspresent in the gap between the two surfaces 60 and 70. An extremelysensitive method of contact potential ionization detection according tothe present invention comprises the steps of generating an electricfield between the surfaces 60 and 70 of the probe 50 and measuring thecurrent using the measurement circuit 80. The electric field ischaracteristic of the contact potential difference. If the gas thatexists between the surfaces 60 and 70 is ionized, then the contactpotential difference, or electric field, attracts the ionized gas. Whenthe gas strikes the surface of the appropriate electrode 24 and 26(including the respective surfaces 60 and 70), a current in themeasurement circuit 80 is recorded. Thus, there is no need to vibrateone of the electrodes 24 and 26. The current is then related to theionization source and the value of the contact potential difference, orthe electrical field between the surfaces 60 and 70.

In yet another form of the invention the probe 50 can be usedadvantageously as a motion detector. The probe 50 operates in the mannerdescribed hereinbefore but the output signal from the measurementcircuit 80 is characteristic not only of contact potential differencebetween the two surfaces 60 and 70 but also a function of motion of themolecules 35 input to the ionization chamber 10 and the resulting ions40 attracted to the surfaces 60 and 70. The voltage output will berelated to the number of gas molecules in the gap (between 60 and 70 inFIG. 3), the rate of ionization of these molecules and the rate at whichthe molecules strike the surfaces 60 and 70. As air or other gasmolecules move into or out of the space between 60 and 70, the voltagewill vary if all other parameters are kept constant. If motion occursoutside of the space between 60 and 70, this will cause gas molecules toflow into the space. In this way, the non-vibrating ionization probe canserve as a motion detector. Such a motion detector would be verysensitive since the probe signals are very sensitive to the motion ofgas molecules and the number of gas molecules. This probe would be moresensitive than currently available motion detectors. Other applicationsof this motion detector would involve the building of a housing toencapsulate the probe so that only specific molecules can enter theregion 60 and 70. Then the probe could discriminate between molecules.

The following non-limiting examples provide examples of variousembodiments of the invention.

Example I

Measurement of the Surface Potential of Unknown Metals or Metals ThatHave Corroded or Otherwise Undergo Changes of Surface Potential

This example demonstrates how the voltage output of the ionization probecan be used to determine the work function of metals whose value is notknown, or whose value has been changed by corrosion or other surfacemodification. This can be done as follows: the voltage output of theionization probe is calibrated against the work functions of selectedmaterials, such as aluminum, copper and tin. The surface of each metalwas polished with an abrasive paper (grit size 2400) and the values ofthe voltage and the known values of the work function are given in theTable below, and these are plotted in FIG. 4.

TABLE Ionization probe voltage output and work function of three examplemetals. Material V_(output)(mV.) φ work function, (V) Copper 180-1854.65 Aluminum  97-101 4.28 Tin 112-118 4.42 Stainless steel 167-172 4.5(deduced from the graph)

In addition to these three metals, a fourth sample of stainless steelwas also polished in the same way. The voltage for the stainless steelwas 167-172 V; and the work function, although unknown, can be deducedto be 4.5 V. In the same way, the work function of other metals andsemiconductors can be found from this calibration curve. In addition, ifthe metal can be exposed to a lubricant, or some other atmosphere, wherethe surface chemistry is changed, work function will become unknown butcan also be obtained. This will also be true if the surface of theunknown metal, semiconductor or other material is corroded or otherwisechemically changed.

Example II

Use of the Ionization Probe as a Gas Flow Meter

The ionization probe can further function as a sensitive gas flow meter.With the probe situated above a metal with a known work function(aluminum, for example) the voltage produced by the circuit will berelated to the ionized gas in the gap between the probe and this metal(schematic of apparatus shown in FIG. 5). If the gas in the gapundergoes movement, then the voltage of the ion probe will change. Thevoltage is then a measure of the movement of the gas molecules in thegap. The sensitivity of the output voltage to gas movement was measuredby an experiment where a gas flow was purposely produced, and the outputvoltage was measured as a function of the flow rate. The ionizationprobe was inserted into a glass tube, one end of which was connected toa dry air gas cylinder. A calibrated flow meter was used to measure theflow of the gas. The measurements of the ion probe voltage (V) and thegas flow rate (ml/min) is shown in FIG. 6. This plot shows that the flowrate can be deduced from measurements of the voltage.

Example III

Ion Probe used as an Accelerometer

The ion probe can also be used as an accelerometer. By arrangement of anexperiment as shown in FIG. 7 the ion probe is positioned above asurface whose spacing, d, and amplitude of vibration can be measured andcontrolled. In these experiments the voltage output of the probe can bedemonstrated to be related to d, the spacing. FIG. 8 shows the voltage(V) versus spacing (mm) for six spacings ranging from 0.5 to 0.75 mm. Ascan be seen in this figure there is a reciprocal relationship betweenthe voltage output and the spacing.

While the invention has been disclosed in its preferred forms, it willbe apparent to those skilled in the art that many modifications,additions, and deletions can be made therein without departing from thespirit and scope of the invention and its equivalents, as set forth inthe following claims.

What is claimed is:
 1. A detector capable of detecting the work functionof a testing surface, said detector comprising: a probe having a firstsurface, said probe having a testing position wherein said first surfaceis positionable proximate to the testing surface, said probe in thetesting position being capable of producing an electric field betweensaid first surface and the testing surface; an ionization source ofionized particles, said ionization source having an ionization sourceoutput, said ionization source arranged such that a portion of theionized particles exit said ionization source output and are capable ofbeing influenced by an electric field generated at said first surface ofsaid probe; and a potential difference measurement circuit capable ofmeasuring a difference in potential between said first surface and thetesting surface when said probe is in the testing position.
 2. Thedetector of claim 1, said first surface being a first conductivematerial.
 3. The detector of claim 2, said first conductive materialbeing different than material of the testing surface.
 4. The detector ofclaim 1, said first surface being maintained at a fixed distance fromthe testing surface when said probe is in the testing position.
 5. Thedetector of claim 1, said potential difference measurement circuitcomprising a power supply circuit that is capable of supplying a firstbias potential to said first surface.
 6. The detector of claim 5, saidpower supply circuit capable of maintaining the testing surface at atesting bias potential approximately equal to ground.
 7. The detector ofclaim 1, said potential difference measurement circuit comprising: acomparator including a first comparator input, a second comparator inputand comparator output, said first comparator input capable of receivinga signal indicative of a first bias potential at said first surface,said second comparator input capable of receiving a signal indicative ofa testing bias potential at said testing surface, said comparator outputcapable of generating a signal indicative of the difference between thesignals indicative of said first bias potential and said testing biaspotential; a first diode including a first diode anode and a first diodecathode, said first diode anode coupled to said first comparator inputand said first diode cathode coupled to said second comparator input;and, a second diode including a second diode anode and a second diodecathode, said second diode anode coupled to said second comparator inputand said second diode cathode coupled to said first comparator input. 8.The detector of claim 1, said probe being a non-vibrating probe.
 9. Adetector capable of detecting measurements reflective of the workfunction of a testing surface, said detector comprising: a non-vibratingprobe having a first conductive material, said probe having a testingposition wherein said first conductive material is positionableproximate to the testing surface, said probe in the testing positionbeing capable of producing an electric field between said firstconductive material and the testing surface; an ionization source ofionized particles, said ionization source having an ionization sourceoutput and said ionization source arranged such that a portion of theionized particles exit said ionization source output and are capable ofbeing influenced by an electric field generated at said first conductivematerial of said probe; and potential difference measurement circuitcapable of measuring a difference in potential between said firstconductive material and the testing surface when said probe is in thetesting position, said potential difference measurement circuitincluding a power supply circuit capable of supplying a first biaspotential to said first conductive material.
 10. The detector of claim9, said first conductive material being maintained at a fixed distancefrom the testing surface when said probe is in the testing position. 11.The detector of claim 9, said power supply circuit capable ofmaintaining the testing surface at a testing bias potentialapproximately equal to ground.
 12. The detector of claim 9, said firstconductive material being a material different than the testing surface.13. The detector of claim 9 wherein said first conductive material isselected from the group consisting of an insulator, an ionic material, asemiconductor and a covalent material.
 14. A potential differencedetector comprising: a non-vibrating probe having a first surface and asecond surface, said first surface being a first conductive material andsaid second surface being a second conductive material; a measurementdevice capable of measuring the potential difference of saidnon-vibrating probe when said probe is provided with ionized gasparticles; and an ionization source of ionized particles, saidionization source being capable of providing ionized gas particles tosaid non-vibrating probe.
 15. The potential difference detector of claim14, said measurement device including a biasing element, said biasingelement being capable of providing a bias potential to saidnon-vibrating probe.
 16. The potential difference detector of claim 14,said first conductive material being different than said secondconductive material.
 17. A motion detector comprising: a firstconductive element for operating at a first bias potential; a secondconductive element for operating at a second bias potential; anionization source having an ionization source input and an ionizationsource output, said ionization source input for receiving gas molecules,said ionization source output capable of providing ionized gas moleculesto said first conductive element and said second conductive element; anda potential difference measurement circuit having a first potentialdifference input, a second potential difference input and a potentialdifference output, said first potential difference input capable ofreceiving a signal indicative of the first bias potential, said secondpotential difference input capable of receiving a signal indicative ofthe second bias potential, said potential difference output generatingan output signal indicative of the difference between the signalsindicative of said first bias potential and said second bias potential,said output signal being a function of motion of the gas molecules insaid motion detector.
 18. The motion detector of claim 17, saidpotential difference measurement circuit comprising: an amplifier havinga first amplifier input, a second amplifier input and an amplifieroutput, said first amplifier input coupled to one of said conductiveelements, said second amplifier input coupled to the other of saidconductive elements; and a voltage source coupled to said amplifieroutput and said first amplifier input.
 19. The motion detector of claim17, said first conductive element and said second conductive elementbeing maintained at a fixed distance with respect to one another.
 20. Amethod of detecting the work function of a testing surface, said methodcomprising the following steps: forming ionized particles; directingsaid ionized particles toward a probe having a first surface being inproximity to the testing surface; generating an electric field betweensaid first surface and the testing surface; and measuring a differencein potential between said first surface and the testing surface.
 21. Themethod of detecting the work function according to claim 20, said stepof forming ionized particles being provided by an ionization source,said ionization source having an ionization source input and anionization source output.
 22. The method of detecting the work functionaccording to claim 21, said step of directing said ionized particlesbeing provided by said ionization source arranged such that a portion ofthe ionized particles exit said ionization source output and are capableof being influenced by the step of generating the electric field. 23.The method of detecting the work function according to claim 20, saidstep of measuring a difference in potential being provided by apotential difference measurement circuit.
 24. The method of detectingthe work function according to claim 23, said potential differencemeasurement circuit comprising a power supply circuit that is capable ofsupplying a first bias potential to said first surface.
 25. The methodof detecting the work function according to claim 24, said power supplycircuit capable of maintaining the testing surface at a testing biaspotential approximately equal to ground.
 26. The method of detecting thework function according to claim 20, further comprising a step ofmaintaining said first surface at a fixed distance from the testingsurface.
 27. The method of detecting the work function according toclaim 20, said probe being a non-vibrating probe.